d through all these
stages."

Simplicity of Early Structures.--The first structures formed are
exceedingly simple in form. It is only by slow degrees that the great
complicity which characterizes many organs is finally attained. For
example, the heart is at first only a straight tube. By enlargement
and the formation of longitudinal and transverse partitions, the fully
developed organ is finally produced. The stomach and intestines are
also at first but a simple straight tube. The stomach and large
intestine are formed by dilatation; and by a growth of the tube in length
while the ends are confined, the small intestines are formed. The other
internal organs are successively developed by similar processes.

The Stages of Growth.--At first insignificant in size--a simple cell,
the embryonic human being steadily increases in size, gradually
approximating more and more closely to the human form, until, at the
end of about nine calendar months or ten lunar months, the new
individual is prepared to enter the world and begin a more independent
course of life. The following condensation of a summary quoted by Dr.
Austin Flint, Jr., will give an idea of the size of the developing being
at different periods, and the rate of progress:--

At the end of the third week, the embryon is a little less than
one-fourth of an inch in length.

At the end of the seventh week, it is three-fourths of an inch long.
The liver, lungs, and other internal organs are partially formed.

At the eighth week, it is about one inch in length. It begins to look
some like a human being, but it is impossible to determine the sex.

At the third month, the embryon has attained the length of two to two
and one-half inches. Its weight is about one ounce.

At the end of the fourth month, the embryon is called a fetus. It is
from four to five inches long, and weighs five ounces.

At the fifth month, the fetus is nearly a foot long, and weighs about
half a pound.

At the sixth month, the average length of the fetus is about thirteen
inches, and its weight one and a half to two pounds. If born, life could
continue a few minutes.

At the seventh month, the fetus is from fourteen to fifteen inches long,
and weighs two to three pounds. It is now viable (may live if born).

At the eighth month, the length of the fetus is from fifteen to sixteen
inches, and its weight from three to four pounds.

At the ninth month, the fetus is about seventeen inches long,